The present invention relates to a method for coloring or tinting photochromic glass, and more particularly to a method for treating photochromic glasses to modify the coloration induced therein by thermal reduction treatments.
Photochromic glasses may be generally characterized as glasses which darken upon exposure to actinic radiation, e.g., ultraviolet light, and which fade in color upon the removal of the activating light. Such glasses have been widely commercialized for the manufacture of ophthalmic lens blanks which will darken under bright outdoor conditions and fade in shade or indoors. Sunglass lenses have also been made of photochromic glasses of this type.
All of the commercially important photochromic glasses are glasses which contain a precipitated microcrystalline silver halide phase. It is this phase which is considered to cause the reversible darkening of the glass under exposure to light. U.S. Pat. No. 3,208,860 provides the basic description of this family of glasses. Subsequent work has resulted in the development of many new families of photochromic glasses exhibiting faster darkening and/or fading response. U.S. Pat. No. 4,190,451, for example, provides a description of some recently developed photochromic glasses of this type.
Photochromic glasses exhibiting a fixed color or tint in the undarkened state have also been commercially developed. Such products have included glasses containing conventional glass colorants, as well as glasses wherein coloration is imparted by treatment of the glass after manufacture with a coloring surface treatment, rather than by the inclusion of glass colorants in the composition. U.S. Pat. Nos. 3,892,582 and 3,920,463, for example, disclose thermal reduction treatments useful for imparting yellow surface colors to photochromic glasses, while U.S. Pat. No. 4,240,836 describes a modified thermal reduction treatment which permits the development of a broader range of colors in this type of glass.
A coloration mechanism which has been postulated for these thermal reduction treatments is the growth of light-absorbing silver metal particles on or near the silver halide crystallites in the glass. Changes in the size and/or configuration of these particles may be responsible for the variations in coloration which are observed.
The effects of optical bleaching on photochromic glasses have long been of interest. U.S. Pat. No. 4,125,404, for example, describes a color adaptation effect observed when a photochromic glass article darkened by exposure to actinic radiation is coincidentally bleached with colored light. The original darkened color of such a glass, which tends to be neutral due to broad-band absorption of the darkened photochromic phase, is permanently modified by such a bleaching treatment such that the darkened color approaches the color of the bleaching radiation.
Optical bleaching has also been employed to modify the color observed in silver halide-containing glasses exhibiting "additive coloration," i.e., color attributable to the presence of permanent absorbing silver particles in the glass. U.S. Pat. No. 4,125,405 describes additively colored glasses and optical bleaching treatments which may be used to permanently modify the color thereof. These glasses may be photochromic glasses or simply colored glasses containing both silver halide and an additional metallic silver coloring phase.
A discussion of the proposed mechanism of color modification in these glasses is provided by N. F. Borrelli et al., "Photoinduced Optical Anisotrophy and Color Adaptation in Silver-Containing Glasses," Appl. Phys. Lett., 34 (6) pages 375-397 (1979). A typical additively colored glass described in that study, which demonstrates strong response to optical bleaching treatments of the kind described, is a silver halide-containing photochromic glass which has been permanently colored by a silver ion-exchange treatment to exchange silver into a surface layer on the glass.
A marked similarity can be observed between photochromic glasses colored by a silver ion exchange treatment as above described and photochromic glasses colored by thermal reduction treatments. The two surface-colored glasses exhibit very similar optical absorption spectra, presumably indicating that a similar silver structure is present in both types of glasses.
Surprisingly, however, optical bleaching treatments which are effective to modify the surface colors of additively-colored (e.g. ion-exchanged) glasses have not been found effective to bleach or otherwise usefully alter the color of glasses which have been tinted or colored using thermal reduction treatments. This is particularly unexpected in view of the probability that the observed coloration in both cases is due to the presence of light-absorbing silver particles on or near silver halide crystallites in the glass.
It is therefore a principal object of the present invention to provide a method for modifying the surface coloration of photochromic glasses wherein the surface color has been generated by thermal reduction treatments.
Other objects and advantages of the invention will become apparent from the following description thereof.